Giant Australian cuttlefish come together to breed along a small stretch of rocky coastline in northern Spencer Gulf, South Australia, during the winter months. This represents the only known breeding aggregation of cuttlefish in the world. Following recent declines in abundance of this iconic population, we were interested to see whether this decline was part of a natural oscillating cycle. We hypothesized that if other cephalopods showed oscillating patterns in abundance then perhaps this may suggest that the decline in abundance of the giant Australian cuttlefish was also part of a natural cycle.
Using historic data, we assembled a global-scale dataset of cephalopod abundance spanning the years 1953 to 2013. We collated 67 time series of catch rate data (catch data standardized to fishing or survey-sampling effort) that spanned 10 years or more. These data represented all major oceanic regions along with key groups and were sourced from both fishery statistics and scientific research surveys. Once data were collected, we coordinated a group of international researchers to be part of a workshop investigating trends in the data. We used statistical models (generalized additive mixed models) that allowed us to determine whether cephalopods were increasing in a linear or non-linear fashion.
Rather than a cyclical pattern in abundance, we found that cephalopod populations have increased over the last six decades. This trend was staggeringly consistent across diverse groups of cephalopods, as well as for both fisheries and survey data. These results suggest that the global proliferation of cephalopods has been driven by large scale processes, common to a range of coastal and oceanic environments and aided by life history characteristics common to all cephalopods.
Although drivers of change were not investigated in our study, plausible causes of the increases include climate change, especially ocean warming (provided there is sufficient food and thermal limits of the cephalopod is not exceeded) as elevated temperatures may accelerate their life cycles. An alternative (or additive) driver may be overfishing and the gradual, global depletion of fish stocks; fish both compete with and prey upon cephalopods. A range of other environmental factors such as climate cycles and eutrophication, as well as habitat modification, may also give a competitive advantage to cephalopods over longer-lived, slower growing marine species such as fish.
Since we commenced this study, there is also good news for giant Australian cuttlefish! The numbers on the breeding aggregation increased in both 2014 and 2015. While it is a little early for abundance estimates for 2016, this year also looks promising.
Image: Squid - Weeds of the sea. Image copyright: Brian Skerry (used with permission)